As a hollow product for allowing fluid to flow therein, an exhaust system for flowing and discharging exhaust gas, or the fluid, is connected to a combustion engine such as an internal combustion engine for a vehicle. As shown in FIG. 19, the exhaust system comprises hollow members such as an exhaust manifold 1, a catalyst converter 2, a pipe 3, a pre-muffler 4 and a main muffler 5. The catalyst converter 2 purifies the exhaust gas discharged from a combustion engine and which flows from the exhaust manifold by causing chemical reactions such as oxidation and reduction of the exhaust gas to take place. The pre-muffler 4 and the main muffler 5 bring high-pressure exhaust gas flowing from the catalyst converter 2 and the pipe 3 close to atmospheric pressure for reducing noise.
The hollow members such as the exhaust manifold 1, the catalyst converter 2, the pipe 3, the pre-muffler 4 and the main muffler 5 are detachably connected to an adjacent hollow members so that the hollow member can easily be replaced by another member when it is damaged. Therefore, the hollow members are provided at ends of containers or connection pipes with flange members capable of being fastened by means of bolts, and thus the hollow members are formed into one unit.
FIG. 20 shows related art for joining a flange member 11′ as one hollow member to the other hollow member 10′ such as a connection pipe. In this related art, a plate is formed with a hole to form the hollow flange member 11′, the hollow member 10′ such as the connection pipe is inserted through the flange member 11′, and the two members are joined to each other by cladding welding.
For example, Japanese Patent Applications Laid-open No. 9-234877, No. 10-29077, No. 9-242540 and No. 63-132790 disclose a technique for joining ends of pipe-like hollow members in their longitudinal direction by friction welding.
This Japanese Patent Application Laid-open No. 63-132790 discloses a manufacturing method for a rotation shaft wherein an end surface of a pipe-like rotation shaft constituent member is friction welded to an end surface of another pipe-like rotation shaft constituent member so as to form a rotation shaft body. In this method, the rotation shaft constituent members are friction welded to each other in a state where a core member for preventing generation of burrs is inserted into the pipes of the rotation shaft constituent members, and. This publication describes that this core member is formed such that its outer diameter is substantially the same as an inner diameter of the rotation shaft constituent member. The publication also describes that the core member is inserted into the pipe of the rotation shaft constituent member which is held in its stationary state before the friction welding operation.
Japanese Patent Publication No. 2957163 discloses an exhaust system part of a double heat insulation structure comprising an inner pipe whose opposite ends are reduced in diameter in a tapered state and an outer pipe whose opposite ends are reduced in diameter in a tapered state, wherein the inner and outer pipes are disposed with a gap interposed therebetween. This publication also discloses a manufacturing method of the exhaust system part. In this publication, there are described that “tip ends of both tapered diameter-reduced portions of the outer pipe are extended by a spinning roller to push the tip ends against an outer peripheral surface of a connecting portion of the inner pipe for plastically deforming the tip ends, and the extended connecting portion and a connecting portion of the inner pipe are connected to each other, thereby integrally forming the inner and outer pipes,” or that “a shock absorbing member is interposed between steps formed on connecting portions of the inner and outer pipes by drawing, thereby preventing the inner and outer pipes from coming into contact with each other by oscillation, the inner and outer pipes are allowed to slide relatively in their axial direction so that a stress caused by difference in temperature, and therefore in coefficient of thermal expansion, between the inner and outer pipes is moderated to enhance the durability” (paragraph numbers are 0024, 0029 and 0030).
Among the above conventional techniques, the one shown in FIG. 20 for cladding welding the plate materials constituting the flange members and the pipe inserted into the holes of the flange members has a problem that a weight of the hollow product is increased due to the cladding by welding. This problem is serious especially in the case of an exhaust system of a moving body having an internal combustion engine such as an automobile. Further, in this related art, it is necessary to take protective measures such as a cover for protecting the hollow member and a welding apparatus from spatter generated at the time of cladding by welding, and there is a problem that equipment cost is increased. Furthermore, in this related art, when the hollow product is used for allowing fluid to flow therein, in order to reliably secure the air-tightness against fluid flowing in the hollow product, it is necessary to continuously carry out the cladding by welding over the entire periphery, and there is a problem that joining quality by highly professional welding is required. Further, there is a problem that the quality of the joined hollow members is varied due to thermal effect by the welding.
Further, among the above conventional techniques, the one for joining the pipes by friction welding has a problem that the joining quality is unstable when end surfaces of relatively thin hollow members are joined. As shown in FIG. 21, in Japanese Patent Applications Laid-open No. 9-234877, No. 10-29077 and No. 9-242540, a burr is generated not only on outer sides of hollow members 30′ and 31′, but also on inner sides thereof due to the friction welding. When the hollow product is used for allowing the fluid therein to carry out a predetermined processing, since the burr generated on the inner side projecting therefrom becomes a flow resistance of the fluid deteriorating the flow efficiency, it is necessary to remove the burr. However, as described in Japanese Patent Application Laid-open No. 63-132790 also, it is difficult to remove the burr generated on the inner side. Therefore, in the related art for joining the pipes by friction welding, it was not possible to enhance the flow efficiency of fluid flowing in the hollow member. The core member in Japanese Patent Application Laid-open No. 63-132790 is merely inserted into the pipe of the rotation shaft constituent member, and there is no description concerning whether the core member is pulled out from the rotation shaft constituent member after the friction welding of the rotation shaft constituent members, or the core member is left therein.
Among the above conventional techniques, in Japanese Patent Publication No. 2957163, the connecting portion of the outer pipe is connected to the connecting portion of the inner pipe directly or through a shock absorbing member merely by drawing using the spinning roller. Therefore, there is a problem that the connection between the inner and outer pipes is weak.